Inter-machine validation study of neoclassical transport modelling in medium- to high-density stellarator-heliotron plasmas

Dinklage, A.; Yokoyama, M.; Tanaka, K.; Velasco, J. L.; López‐Bruna, D.; Beidler, C. D.; Satake, S.; Ascasíbar, E.; Arévalo, Juan Miguel Insúa; Baldzuhn, J.; Feng, Y.; Gates, D.; Geiger, J.; Ida, K.; Isaev, M. Yu.; Jakubowski, M.; Löpez‐Fraguas, A.; Maaßberg, H.; Miyazawa, J.; Morisaki, T.; Murakami, S.; Pablant, N.; Kobayashi, S.; Seki, R.; Suzuki, C.; Suzuki, Y.; Turkin, Y.; Wakasa, A.; Wolf, R. C.; Yamada, H.; Yoshinuma, M.; Team, W As

doi:10.1088/0029-5515/53/6/063022

Cited by 49 publications

(58 citation statements)

References 30 publications

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“…In this paper we study stellarators close to omnigeneity in the collisionality regime (4), relevant for a stellarator reactor [18]. It is important to point out that the calculations in this paper do not rely on large aspect ratio approximations.…”

Section: Introductionmentioning

confidence: 99%

The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity

Calvo¹,

Parra

Velasco³

et al. 2017

Plasma Phys. Control. Fusion

205

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Abstract. In general, the orbit-averaged radial magnetic drift of trapped particles in stellarators is non-zero due to the three-dimensional nature of the magnetic field. Stellarators in which the orbit-averaged radial magnetic drift vanishes are called omnigeneous, and they exhibit neoclassical transport levels comparable to those of axisymmetric tokamaks. However, the effect of deviations from omnigeneity cannot be neglected in practice, and it is more deleterious at small collisionalities. For sufficiently low collision frequencies (below the values that define the 1/ν regime), the components of the drifts tangential to the flux surface become relevant. This article focuses on the study of such collisionality regimes in stellarators close to omnigeneity when the gradient of the non-omnigeneous perturbation is small. First, it is proven that closeness to omnigeneity is required to actually preserve radial locality in the drift-kinetic equation for collisionalities below the 1/ν regime. Then, using the derived radially local equation, it is shown that neoclassical transport is determined by two layers located at different regions of phase space. One of the layers corresponds to the so-called √ ν regime and the other to the so-called superbanana-plateau regime. The importance of the superbanana-plateau layer for the calculation of the tangential electric field is emphasized, as well as the relevance of the latter for neoclassical transport in the collisionality regimes considered in this paper. In particular, the role of the tangential electric field is essential for the emergence of a new subregime of superbanana-plateau transport when the radial electric field is small. A formula for the ion energy flux that includes the √ ν regime and the superbanana-plateau regime is given. The energy flux scales with the square of the size of the deviation from omnigeneity. Finally, it is explained why below a certain collisionality value the formulation presented in this article ceases to be valid.

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Section: Introductionmentioning

confidence: 99%

The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity

Calvo¹,

Parra

Velasco³

et al. 2017

Plasma Phys. Control. Fusion

205

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“…The lack of temperature screening in typical stellarator scenarios is caused by the fact that both the radial electric field E r and the ion temperature gradient T i contribute to radial impurity transport; the latter, through an outward pinch roughly given by T i ; the former, by means of and inward pinch Z I eE r (Z I e being the charge of the impurity and e the proton charge). Even for peaked ion temperature profiles, in standard ion-root conditions [11] the radial electric field is bound to be as large as the ion temperature gradient, eE r ∼ T i , and Z I 1 ensures impurity accumulation. The large charge number Z I anticipates one of the additional physical mechanisms that is a candidate for explaining how impurities are flushed out from the plasma despite the negative radial electric field (that is, the exceptions mentioned above): the electrostatic potential is only approximately a constant on the flux surfaces of a stellarator [5,6,12] and, for low collisionalities of the bulk species, its variation on the flux surface, that we denote by ϕ 1 (in other words, the component of the electric field that is tangent to the flux surface), can in principle be a relevant drive for the radial transport of moderate-to-high Z I impurities.…”

Section: Introductionmentioning

confidence: 99%

Large tangential electric fields in plasmas close to temperature screening

Velasco¹,

Calvo²,

García-Regaña³

et al. 2018

Plasma Phys. Control. Fusion

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Low-collisionality stellarator plasmas usually display a large negative radial electric field that has been expected to cause accumulation of impurities due to their high charge number. In this paper, two combined effects that can potentially modify this scenario are discussed. First, it is shown that, in low collisionality plasmas, the kinetic contribution of the electrons to the radial electric field can make it negative but small, bringing the plasma close to impurity temperature screening (i.e., to a situation in which the ion temperature gradient is the main drive of impurity transport and causes outward flux); in plasmas of very low collisionality, such as those of the Large Helical Device displaying impurity hole [1,2], screening may actually occur. Second, the component of the electric field that is tangent to the flux surface (in other words, the variation of the electrostatic potential on the flux surface), although smaller than the radial component, has recently been suggested to be an additional relevant drive for radial impurity transport. Here, it is explained that, especially when the radial electric field is small, the tangential magnetic drift has to be kept in order to correctly compute the tangential electric field, that can be larger than previously expected. This can have a strong impact on impurity transport, as we illustrate by means of simulations using the newly-developed code KNOSOS (KiNetic Orbit-averaging-SOlver for Stellarators).

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“…In addition, neoclassical transport predicts hollow density profiles if the temperature profiles are sufficiently peaked [26], [27]. Therefore, a suitable fueling scheme will have to be established.…”

Section: Development Of Integrated Steady-state Plasmas (Op2)mentioning

confidence: 99%

Wendelstein 7-X Program—Demonstration of a Stellarator Option for Fusion Energy

Wolf

Beidler

Dinklage

et al. 2016

IEEE Trans. Plasma Sci.

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The superconducting stellarator Wendelstein 7-X is currently being commissioned. First plasmas are expected for the second half of 2015. W7-X is designed to overcome the main drawbacks of the stellarator concept and simultaneously demonstrate its intrinsic advantages relative to the tokamaki.e., steady-state operation without the requirement of current drive or stability control. An elaborate optimization procedure was used to avoid excessive neoclassical transport losses at high plasma temperature, while simultaneously achieving satisfactory equilibrium and stability properties at high β in combination with a viable divertor concept. In addition, fastion confinement must be consistent with the requirements of alpha-heating in a power plant. Plasma operation of Wendelstein 7-X follows a staged approach following the successive completion of the in-vessel components. The main objective of Wendelstein 7-X is the demonstration of steady-state plasma at fusion relevant plasma parameters. Wendelstein 7-X will address major questions for the extrapolation of the concept to a power plant. These include divertor operation at high densities, plasma fueling at high central temperatures, avoiding impurity accumulation, and an assessment of the effect of neoclassical optimization on turbulent transport and fast-ion confinement. A power plant concept based on an extrapolation from Wendelstein 7-X, the helical advanced stellarator, has been developed.Index Terms-Fusion power plant, steady-state magnetic confinement, stellarator.

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Inter-machine validation study of neoclassical transport modelling in medium- to high-density stellarator-heliotron plasmas

Cited by 49 publications

References 30 publications

The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity

The effect of tangential drifts on neoclassical transport in stellarators close to omnigeneity

Large tangential electric fields in plasmas close to temperature screening

Wendelstein 7-X Program—Demonstration of a Stellarator Option for Fusion Energy

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